Hydrocarbons Notes: Class 11 Chemistry Chapter 9
Class 11 Chemistry Chapter 9 Hydrocarbons covers the classification, nomenclature, preparation, properties, and reactions of alkanes, alkenes, alkynes, and aromatic hydrocarbons (arenes).
Overview of Topics & Subtopics (Click to Jump)
1. Alkanes
Short Note:
• Alkanes are saturated hydrocarbons with general formula CnH2n+2, containing only C–C and C–H single bonds.
• Prepared by Wurtz Reaction (2RX + 2Na → R–R + 2NaX) and undergo characteristic reactions such as free radical halogenation and combustion.
2. Alkenes
Short Note:
• Alkenes are unsaturated hydrocarbons containing at least one C=C double bond, general formula CnH2n.
• Undergo characteristic electrophilic addition reactions, such as addition of H2 (hydrogenation), halogens, HX (following Markovnikov's Rule), and water.
3. Alkynes
Short Note:
• Alkynes contain at least one C≡C triple bond, general formula CnH2n−2; the simplest member, ethyne (acetylene), is prepared from calcium carbide and water.
• Undergo addition reactions similar to alkenes but generally require two equivalents of the reagent (since two pi bonds must be broken).
4. Aromatic Hydrocarbons (Arenes)
Short Note:
• Benzene (C6H6) is the simplest aromatic hydrocarbon, characterised by a ring of alternating single/double bonds with delocalised pi electrons, giving it special stability (aromaticity).
• Arenes primarily undergo electrophilic substitution reactions such as nitration, sulphonation, halogenation, and Friedel-Crafts alkylation/acylation, rather than addition reactions, due to the stability of the aromatic ring.
NCERT Textbook Exercise Questions
Write the IUPAC name and structure of the product formed by the addition of HBr to propene, according to Markovnikov's rule.
Answer: Propene: CH3–CH=CH2. According to Markovnikov's rule, H adds to the carbon with more hydrogens (terminal CH2) and Br adds to the carbon with fewer hydrogens (middle carbon), giving CH3–CHBr–CH3, i.e. 2-bromopropane as the major product.
How would you convert ethyne into benzene?
Answer: Passing ethyne (acetylene) through a red-hot iron tube at 873 K causes cyclic polymerisation of three molecules of ethyne to form one molecule of benzene (C6H6).
Why does benzene undergo electrophilic substitution reactions more readily than addition reactions?
Answer: Benzene's delocalised pi electron cloud gives it extra stability (aromaticity/resonance energy). Addition reactions would disrupt this stable delocalised system, whereas substitution reactions preserve the aromatic ring, making substitution energetically more favourable.
Arrange ethane, ethene and ethyne in increasing order of acidic character, and give a reason.
Answer: Increasing order: ethane < ethene < ethyne. This is because the terminal C–H bond in ethyne involves an sp-hybridised carbon, which has greater s-character (50%) than sp² (33%) in ethene or sp³ (25%) in ethane; greater s-character means the electrons are held closer to the nucleus, making the C–H bond more polar and the hydrogen more acidic.
What is Markovnikov's rule? Explain with an example.
Answer: Markovnikov's rule states that when an unsymmetrical reagent (like HX) is added to an unsymmetrical alkene, the negative part (X) of the reagent attaches to the carbon atom having fewer hydrogen atoms, while H attaches to the carbon with more hydrogen atoms. For example, addition of HCl to propene (CH3–CH=CH2) gives 2-chloropropane (CH3–CHCl–CH3) as the major product.
